1. Field of the Invention
The present invention relates to a zoom lens system, which is to be used mainly in an electronic still camera (digital camera), having a zoom ratio (magnification ratio) exceeding 4 and including a wide angle range.
2. Description of the Prior Art
In recent years, in order to meet the increased need for further miniaturization and higher precision/density of cameras, and higher density of the pixels in CCD image devices are being achieved. Accordingly, a photographing lens system for a digital camera is required to have high resolution. Furthermore, a long back focal distance is also required in order to accommodate a filter group. In an optical system for a color CCD, in order to prevent shading and color shift, good telecentricity, in which the light exits from the final lens surface of the photographing lens system is made incident on the imaging surface at an angle as close to a right angle (90°) as possible, is required.
As a miniaturized zoom lens system for a compact digital camera, it is possible to apply a negative-lead type lens system for a zoom ratio of up to 3 or 4. In a negative-lead type lens system, an increased wide-angle (i.e., a shorter focal length) at the short focal length extremity and further miniaturization of the lens system (especially with respect to reduction of the front lens diameter) can be achieved. Accordingly, such a lens system is suitable for a retractable zoom lens system in which the distance between the lens groups is shortened so that the zoom lens barrels can retract inwards (towards the camera body). However, in such a zoom lens system (negative-lead type lens system), it is difficult to achieve a zoom ratio of 4 or more.
United States Patent (hereinafter, USP) U.S. Pat. No. 5,100,223 and U.S. Pat. No. 5,570,233 are examples of zoom lens systems having a zoom ratio of 4 or more. A zoom ratio of about 6 is achieved in U.S. Pat. No. 5,100,223 and a zoom ratio of about 10 is achieved in U.S. Pat. No. 5,570,233; however, in both publications, the diameter of the front lens group is large, and miniaturization of the camera is not sufficiently attained.
Furthermore, in U.S. Pat. No. 5,100,223, the half angle-of-view at the short focal length extremity is about 25°, which means that a wide angle-of-view, i.e., the shorter focal length at the short focal length extremity, is not sufficiently achieved. U.S. Pat. No. 5,570,233 achieved a wide angle of view; however, the front lens diameter is large, and employs a large number of lens elements, which is not suitable for a retractable zoom lens system for a camera.
In order to further miniaturize a camera with a retractable zoom lens system employing a multi retractable lens barrels, it is necessary to simplify the mechanical construction for moving the movable lens groups upon zooming. Generally, if the number of lens groups is reduced, the mechanical construction therefor can be simplified; however, it is difficult, on the other hand, to achieve a higher zoom ratio. Furthermore, in order to further miniaturize the camera body, a small front lens diameter and a short overall length of the zoom lens system are also necessary. Consequently, a reduced thickness of each lens group is required.
However, if the number of lens elements is reduced in order to miniaturize the zoom lens system, and if the thickness of the lens group is reduced, the correcting of aberrations becomes increasingly difficult. In other words, adequate distribution of refractive power over each lens group, and a specific lens arrangement for this purpose are required.